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Weathering changes in aerodynamic innovations

Its rugged construction makes the Helipod ideal for data capture in extreme temperatures and environments, such as on this icebreaker in the Artic.

Its rugged construction makes the Helipod ideal for data capture in extreme temperatures and environments, such as on this icebreaker in the Artic.

The compact Helipod, seen here with a recent LITFASS field crew, can be easily disassembled and transported.

Most people are familiar with the expression, “Trying to build a better mousetrap,” and yet no one has really been able to do it. A design so simple that it accomplishes its intended purpose seems to leave no room for improvement. However, not many innovations can lay claim to perfection on the first go-around.

What we tend to see, more often than not, is that an invention’s design principles serve as the basis to take that invention into new realms of possibilities and technological advancements. Such is the case with the Helipod, an autonomous measuring probe attached to a helicopter in such a way that it is out of the downwash area of the carrying helicopter.

The probe, developed in the mid-1990s, recently underwent a technological refresh and modernization by the Institute of Aerospace Systems (ILR) of the University of Braunschweig, Germany. The Helipod’s main purpose is to measure basic meteorological quantities, not only on a wide scale but including extremely small variations—i.e. small-scale turbulence. Among others, the gathered measuring data helps to understand the energy exchange between the atmosphere and the Earth’s surface in order to improve numerical models. This also has an influence on the quality of our daily weather forecast.

The system meets extreme environmental conditions using standardized, inexpensive hardware and software. For temperature measurement, the unit includes a Rosemount 102 platinum resistance wire thermometer as well as an Aerodata AD-STS with a Dantec “open-wire” element. The Helipod’s humidity sensor system consists of a dew point mirror, a Humicap capacitive sensor, and a Lyman-Alpha hygrometer. Only a few meters in length, the Helipod probe is controlled via a complex measuring computer system based on CompactPCI boards and M-Modules from MEN Micro for harsh environmental conditions that collects and analyzes the measurement data.

The Helipod is the most modern airborne system worldwide for measuring atmospheric turbulences. In addition to its operation in many applications in Germany, such as LITFASS (Lindenberg Inhomogeneous Terrain - Fluxes between Atmosphere and Surface), the Helipod has been used in the Arctic on the Polarstern, a 17,300-ton polar icebreaker operated by Germany’s Alfred Wegener Institute, for the PHELIX (Profiler-HELIPOD Intercomparison Experiment) project at California’s Vandenberg Air Force Base on the Pacific Coast near Santa Maria, and in other applications with many notable international scientific organizations. Aside from uniquely precise measuring results, these missions yielded some unexpected scientific findings in the lower atmosphere. Often, a number of measuring types (such as micro-meteorological ground stations and remote sensing systems) are carried out over the same area, with the highly accurate and geographically precise Helipod probe data being taken as a reference.

The Helipod drag probe has a length of 5 m, a diameter of 600 mm, and a mass of about 250 kg. The probe is populated with measuring equipment that is controlled by an industrial computer from MEN Micro. During the measurements, the probe is attached to a helicopter using a 15-m-long rope. At a forward speed of 40 m/s, the air turbulence caused by the helicopter rotor is driven to the rear so strongly that the probe is not affected by it. With this relatively small forward speed, the probe can log measurements using the latest instruments and the powerful onboard computer at a precision unattainable by other measuring systems (400 mm after anti-aliasing filter). Since the probe does not have wings, propellers, or an engine, the undisturbed state of the air is measured in unaltered form at the current parameters.

The probe records turbulent transport, wind vector, humidity, air and surface temperatures, as well as CO2 at very high precision in defined altitudes up to 2000 m above land or water surface. Most flights follow a grid pattern and accurately acquire the dynamic state of the atmosphere. The integration of further measuring instruments as well as optical and infrared cameras is optional.

All measurements are done using two different devices. One device measures fast, but drifts in time. The other device measures slowly but very precisely. The two data sets are merged for analysis using complementary filters. This makes for the precision previously unattainable, as noted earlier (400 mm).

An unexpected result lay in the measurements taken at high altitudes due to the air’s high heterogeneity. Up to now, scientists believed that air was mixed at heights of 100 to 500 m. However, Helipod measurements have shown that air keeps its structure at even greater heights depending on the surface underneath (forest, farmland, lake, sea, ice). Air mixes completely with neighboring air over these types of surfaces only in much higher regions, influencing weather-forecasting models.

The Helipod is a self-sufficient system with its own power supply using batteries, navigation systems, data processing, and mass storage. The basis of the onboard computer system is a robust industrial computer from MEN Micro in a standardized CompactPCI format. The probe’s most important feature is the ability to operate over a wide temperature range—from -40°C in the Arctic to up to +85°C in the desert. This conforms to the E2 industrial standard temperature range and the T2 telecom temperature standard range. A 300-MHz MPC8245 PowerPC processor with a 603e core sufficiently controls the system. The 6U computer board’s total power dissipation is approximately 8 W. Low power consumption with high logical performance is critical for applications with demanding temperature ranges due to the limited capacity of the onboard batteries.

For the Helipod project, the single-board computer is populated with SDRAM, flash memory, and a CompactFlash card. Four serial interfaces, one USB, and two ethernet ports are also included. In addition, the SBC can carry up to three M-Modules (mezzanine modules) according to the ANSI/VITA-12 standard. Up to four additional M-Modules can be accommodated on a passive carrier board. For this project, an I/O module with 32 individually usable digital inputs/outputs, four 16-bit A/D modules with 32 differential inputs, and a 12-bit A/D module with 16 single-ended inputs are used, for a total of six M-Modules. Another M-Module provides the ARINC interface that transfers the position data to the CompactPCI system. Thus, only two CompactPCI boards, in an exceptionally small housing, are needed for data processing and for the connection of multiple measuring inputs and control outputs. To precisely determine its position, Helipod has several onboard GPS receivers. All data is intermediately stored in a very large flash memory for even more precise filtering and analysis later on in the laboratory.

The computer system runs the ELinOS embedded Linux system from Sysgo and uses the integrated RTAI for real-time requirements. Because of the special requirements, ILR has made a number of adaptations in the Linux core. Owing to its open source, this is no problem with Linux. Drivers and board support packages for all plug-on boards are easily available.